The present invention relates to an improved valve for fluid flow systems. In particular, the present invention is concerned with an improved wheel valve for central tire inflation systems (CTI systems), also known as on-board inflation systems and traction systems, wherein the inflation pressure of vehicle tires may be monitored and controlled from a remote location on the vehicle with the vehicle at rest or in motion.
Various types of systems have been suggested in the past for controllably inflating and deflating vehicle tires during vehicle operation. Such systems typically include a source of air under pressure and controls for selectively increasing or decreasing an existing tire pressure, and for checking to determine the existing tire pressure for each tire. The ability to selectively increase or decrease tire pressure is desirable in connection with optimizing the operation of the vehicle under widely changing conditions including weather, vehicle load, terrain and vehicle speed. It is also desirable to provide adaptability for isolating the air under pressure in each vehicle tire from the remainder of the system so that a problem such as a leak in one tire does not affect the air pressure in the other tires. Further, in connection with such a tire isolating feature, an air line from the system's source of air under pressure extends to a vehicle tire generally through the fixed axle upon which the tire and its associated wheel assembly are rotatably mounted and across a sealing arrangement between the fixed and rotating parts. The tire isolating wheel valve is preferably between the sealing arrangement and tire so that the sealing arrangement is not subjected to system air under pressure other than at times of operation of the system to accomplish inflation, deflation or pressure checking.
CTI systems are well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 5,253,687; 5,174,839; 5,273,064; 4,619,303; 4,754,792; 4,782,879; 4,825,925; 4,860,579; 4,877,048; 4,883,105; 4,893,664; 4,898,216; 4,917,163; 4,922,946; and 4,924,926. The entire disclosure of each of these patents is incorporated herein by reference.
Generally, the prior art CTI systems have employed a pneumatically controlled wheel valve that is affixed to each vehicle wheel assembly for effecting tire pressure inflation/deflation in response to pressure signals from an air control circuit. The air control circuit is connected to each wheel valve via a rotary seal assembly associated with each wheel valve. Rotary seals are utilized which are located in a well protected inboard location (see U.S. Pat. No. 4,434,833 assigned to the Assignee of this invention), utilizing valves and conduits of a relatively rugged proven construction which may be located at a protected location or inside the vehicle tire to protect same. Wheel valves automatically isolate those tires having less than a predetermined minimum reference pressure from the remainder of the system. Valve operations exhaust internal pressure on the rotary seals during steady state operation of the system.
The above is accomplished by providing a central tire inflation system for a vehicle utilizing a wheel valve assembly comprising a wheel valve and a low tire pressure shutoff valve at each wheel end assembly (usually a single or dual tire) where the valve assembly is connected to the central control system by a single pressure line or conduit through a rotary seal assembly and pressurization of which single pressure conduit is effective to open and close communication to the vehicle tire and to cause inflation and/or deflation of said tire to a selected pressure. The wheel valve assembly may be located exteriorly of the tire or may be located interiorly of the tire such as in the bead lock portion thereof. The low tire pressure shutoff valve is effective to automatically isolate the tire associated therewith from the remainder of the central tire inflation system when the inflation pressure thereof is below a predetermined minimum reference value, or when the system is in an inactive mode.
The single pressure conduit leading to the rotary seal at the wheel hub assembly is connected to a source of pressurized fluid through a plurality of control valves which are effective to open communication to the tire to measure the existing pressurizing thereof, to cause inflating or deflation of the tire as required, to establish or block communication to the tire from the central tire inflation system and to exhaust the single pressure line during steady state conditions of the tire inflation system and to relieve pressure on the rotary seals. Preferably, the control valves, which may be separate or provided in a single valve block, are operated by a control, preferably a microprocessor control, which senses the tire inflation value selected by the operator, senses vehicle velocity and the current pressure condition of the tire pressures, vehicle air brake system and the inflation system reservoir.
Although these prior art CTI systems have functioned well in the past, newer applications for CTI systems have been developed that have the dual requirements of fast deflation rates and the capability of inflating the tires to high pressure levels (75 psig and higher). Although some prior art systems are able to accommodate both of these operating requirements, such systems are undesirably complicated, typically requiring multi-diaphragm valves and/or venting or exhausting at the wheel. Less complicated, single diaphragm valve systems, without venting or exhausting at the wheel, have heretofore been able to completely accommodate only one of these two operating requirements. This is because prior art single diaphragm valve systems have accommodated faster deflation rates by using a wheel valve with a larger seat orifice through which larger volumes of air can be exhausted. However, the widening of the seat orifice causes increased pressure forces which tend to keep the wheel valve open when high pressure shutoff is desired, thus leading to degraded and less accurate shutoff performance and a lowered maximum tire inflation pressure.
Since the rate of deflation varies exponentially with the pressure differential between the tire and the ambient air, the deflation rate slows considerably once lower tire pressures are encountered. To maintain a relatively rapid rate of deflation at these lower tire pressures, the opening of the exhaust valve must be of a relatively large cross-sectional area to permit an outflow of air.
A large cross-sectional area results in large forces when the air pressure is at a high level because the force generated when closing the valve is proportional to the cross-sectional area. Specifically, the force is approximately equal to the air pressure multiplied by the cross-sectional area. Thus, if an exhaust valve having a large cross-sectional area is needed for rapid tire deflation then large forces will be generated when closing off the outlet at higher pressures. Using prior art valves, these forces have exceeded the available closing force and the exhaust valve remains open despite the command to close off. The challenge has been how to obtain rapid deflation at low tire pressures while limiting the pressure induced forces that must be handled in the wheel valve at high pressure.
Another problem with prior art wheel valves is that they are vented to atmosphere. The vent in the wheel valve provides an opening through which contaminants such as dust and/or sand can be drawn into the valve thereby causing operational failure of the valve.
Thus, an improved single diaphragm wheel valve is needed that, without exhausting or venting at the wheel, can accommodate the newer CTI system applications by providing both fast deflation rates and the capability of providing reliable valve shutoff at high pressure levels.